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Lesson Overview. 26.1 Invertebrate Evolution and Diversity. Origins of the Invertebrates. For roughly 3 billion years after the first prokaryotic cells evolved, all prokaryotes and eukaryotes were single-celled.
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Lesson Overview 26.1 Invertebrate Evolution and Diversity
Origins of the Invertebrates • For roughly 3 billion years after the first prokaryotic cells evolved, all prokaryotes and eukaryotes were single-celled. • Animals evolved from ancestors they shared with organisms called choanoflagellates, single-celled eukaryotes that sometimes grow in colonies. • Choanoflagellates share several characteristics with sponges, the simplest multicellular animals.
Traces of Early Animals • Our oldest evidence of multicellular life comes from microscopic fossils that are roughly 600 million years old. • The first animals were tiny and soft-bodied, so few fossilized bodies exist. • Recent studies have uncovered incredibly well preserved fossils of eggs and embryos that are 565-million-years-old.
Traces of Early Animals • Other fossils from this time period have been tentatively identified as parts of sponges and animals similar to jellyfish. • Paleontologists have also identified what are called “trace fossils,” tracks and burrows made by animals whose body parts weren’t fossilized.
The Ediacaran Fauna • Strange fossils, which date from roughly 565 to about 544 million years ago, show body plans that are different from those of anything alive today. • Many of the organisms were flat and lived on the bottom of shallow seas. • They show little evidence of cell, tissue, or organ specialization, and no organization into a front and back end.
The Cambrian Explosion • The Cambrian Period (about 542 million years ago) fossils show that over a period of 10–15 million years, animals evolved complex body plans, including specialized cells, tissues, and organs.
The Cambrian Explosion • A number of Cambrian fossils have been identified as ancient members of modern invertebrate phyla, such as the fossil of arthropod Marrella shown. • Some early Cambrian fossils represent extinct groups so peculiar that no one knows what to make of them.
The Cambrian Explosion • By the end of the Cambrian Period, all the basic body plans of modern phyla had been established. • Later evolutionary changes, which produced the more familiar body structures of modern animals, involved variations on these basic body plans.
Modern Invertebrate Diversity • Today, invertebrates are the most abundant animals on Earth. • Invertebrates live in nearly every ecosystem, participate in nearly every food web, and vastly outnumber so-called “higher animals,” such as reptiles and mammals.
Cladogram of Invertebrates • Groups shown close together are more closely related than are groups shown farther apart. The sequence in which some important features evolved is also shown.
Sponges • Phylum: Porifera (“pore bearers”) • Sponges are the most ancient members of the kingdom Animalia. • They are multicellular, heterotrophic, lack cell walls, and contain a few specialized cells.
Cnidarians • Phylum: Cnidaria—includes jellyfishes, sea fans, sea anemones, hydras, and corals • Cnidarians are aquatic, soft-bodied, carnivorous, radially symmetrical animals with stinging tentacles arranged in circles around their mouths. • They are the simplest animals to have body symmetry.
Arthropods • Phylum: Arthropoda (arthron = “joint,” podos = “foot”)—includes spiders, centipedes, insects, and crustaceans • Arthropods have bodies divided into segments, a tough external skeleton called an exoskeleton, cephalization, and jointed appendages, which are structures such as legs and antennae that extend from the body wall. • Arthropods appeared in the sea about 600 million years ago and have since colonized freshwater habitats, land, and air.
Nematodes (Roundworms) • Phylum: Nematoda • Nematodes are unsegmented worms with pseudocoeloms, specialized tissues and organ systems, and digestive tracts with two openings—a mouth and an anus. • Nematodes were once thought to be closely related to flatworms, annelids, and mollusks but have been found to be more closely related to the arthropods.
Flatworms • Phylum: Platyhelminthes • Flatworms are soft, unsegmented, flattened worms that have tissues and internal organ systems. • They are the simplest animals to have three embryonic germ layers, bilateral symmetry, and cephalization. • Flatworms do not have coeloms.
Annelids • Phylum: Annelida (annellus = “little ring”)—includes earthworms, some marine worms, and leeches • Annelids are worms with segmented bodies and a true coelom lined with tissue derived from mesoderm.
Mollusks • Phylum: Mollusca—includes snails, slugs, clams, squids, and octopi • Mollusks are soft-bodied animals that have an internal or external shell. • They have true coeloms surrounded by mesoderm and complex organ systems. • Many mollusks have a free-swimming larva, or immature stage, called a trochophore.
Echinoderms • Phylum: Echinodermata (echino = “spiny,” dermis = “skin”)—includes sea stars, sea urchins, and sand dollars • Echinoderms have spiny skin and an internal skeleton. • They also have a water vascular system—a network of water-filled tubes that include suction-cuplike tube feet, which are used for walking and gripping prey. • Most exhibit five-part radial symmetry and are deuterostomes.
